Abstract
When an object is placed on a water surface, the air-water interface deforms and a meniscus arises due to surface tension effects, which in turn produces a lift force or drag force on the partly submerged object. This study aims to investigate the underlying mechanism of the vertical force acting on spindly cylinders in contact with a water surface. A simplified 2-D model is presented, and the profile of the curved air-water interface and the vertical force are computed using a numerical method. A parametric study is performed to determine the effects of the cylinder center distance, inclined angle, static contact angle, and radius on the vertical force. Several key conclusions are derived from the study: (1) Although the lift force increases with the cylinder center distance, cylinders with smaller center distances can penetrate deeper below the water surface before sinking, thereby obtaining a larger maximum lift force; (2) An increase in the inclined angle reduces the lift force, which can enable the lower cylinders fall more deeply before sinking; (3) While the effect of static contact angle is limited for angles greater than 90°, hydrophobicity allows cylinders to obtain a larger lift force and load capacity on water; (4) The lift force increases rapidly with cylinder radius, but an increase in radius also increases the overall size and weight of cylinders and decreases the proportion of the surface tension force. These findings may prove helpful in the design of supporting legs of biologically-inspired miniature aquatic devices, such as water strider robots.
Highlights
In nature, some aquatic insects, such as water striders, are able to stand on the water surface due to the lift force created at the curvature of the air-water interface.[1,2,3] Experiments have shown that a single leg of a water strider can provide a lift force on the water surface equivalent to approximately 15 times the total body weight of the insect.[2,4] This phenomenon, which is attributed to superhydrophobicity and the effect of surface tension, has provided inspiration for several biologicallybased miniature water-walking robots
Several key conclusions are derived from the study: (1) the lift force increases with the cylinder center distance, cylinders with smaller center distances can penetrate deeper below the water surface before sinking, thereby obtaining a larger maximum lift force; (2) An increase in the inclined angle reduces the lift force, which can enable the lower cylinders fall more deeply before sinking; (3) While the effect of static contact angle is limited for angles greater than 90◦, hydrophobicity allows cylinders to obtain a larger lift force and load capacity on water; (4) The lift force increases rapidly with cylinder radius, but an increase in radius increases the overall size and weight of cylinders and decreases the proportion of the surface tension force
When cylinder center distance decreases to a certain extent, individual meniscuses begin to merge and integrate with each other, which in turn induce a disturbance in the vertical force acting on the cylinders
Summary
Some aquatic insects, such as water striders, are able to stand on the water surface due to the lift force created at the curvature of the air-water interface.[1,2,3] Experiments have shown that a single leg of a water strider can provide a lift force on the water surface equivalent to approximately 15 times the total body weight of the insect.[2,4] This phenomenon, which is attributed to superhydrophobicity and the effect of surface tension, has provided inspiration for several biologicallybased miniature water-walking robots. When an object comes into contact with a water surface, the air-water interface deforms and a meniscus develops. The air-water interface deformation due to surface tension, which is governed by Young-Laplace equation, plays an important role in the interaction between the water and object, especially for small objects. Keller[12] demonstrated that the vertical force acting on submerged objects, composed of the buoyant force and the vertical component of the surface tension force, is
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